This invention relates to an image processing apparatus and, more particularly, to an image processing apparatus such as one suitable for an image output apparatus such as a printer for outputting input image information upon enlarging the information by zooming, or an apparatus for performing a resolution conversion from low-resolution information to high-resolution information in communication between machine models having different resolutions.
Various methods have been proposed for converting input low-resolution information to high-resolution information. With these proposed conventional methods, however, the methods of conversion processing differ depending upon the type of image dealt with (e.g., a multivalued image having gray-level information for each and every pixel, a two-value image binarized by pseudo-halftones, a two-value image binarized by a fixed threshold value, a character image, etc.).
Methods of interpolation which are conventional methods of converting resolution in multivalued images such as natural pictures having gray-level information for each pixel include nearest neighbor interpolation, as shown in FIG. 25, in which pixel values that are the same as observation points closest to an interpolated point are arrayed, and bi-linear interpolation (linear interpolation), as shown in FIG. 26, in which a pixel value E is decided by calculation (see Equation 1 below) based upon distances of four points (let A, B, C, D represent the pixel values of the four points) that surround the interpolated point. EQU E=(1-i)(1-j)A+i.multidot.(1-j)B+j.multidot.(1-i)C+ijD (Eq. 1)
If the distance between pixels is "1", then it is assumed that there is a distance i from A in the horizontal direction and a distance j from A in the vertical direction (i.ltoreq.1, j.ltoreq.1).
However, the examples of the prior art described above have the shortcomings described below.
Specifically, though the method of FIG. 25 is advantageous in terms of construction, pixel values are decided for each block enlarged in a case where the image dealt with is used in a natural picture or the like. As a consequence, blocks become conspicuous visually, thus resulting in poor picture quality.
In a case where the image dealt with is used in a character, line image or CG (computer graphics), identical pixel values are continuous for each block enlarged. As a result, images such as of slanted lines become poor in quality owing to so-called "jaggies", as shown in FIGS. 27A, 27B. FIGS. 27A-27C are examples of two-fold resolution conversion vertically and horizontally. The greater the magnification, the greater the deterioration in picture quality ("200", "10" in FIGS. 27A-27C are pixel values).
The method of FIG. 26 is generally used in enlarging natural pictures. With this method, an averaged, smoothed image is obtained. However, portions which require edges or sharp quality become blurred. Furthermore, in case of an image obtained by scanning a map or the like or a natural picture which includes characters, there are occasions where important information is not transmitted to the receiving party owing to blurring caused by interpolation.
FIG. 27C illustrates image information obtained by applying interpolation processing by a factor of two in each of the horizontal and vertical directions to the image information shown in FIG. 27A by the method of FIG. 26.
FIG. 27C clearly shows that slanted lines themselves and not just the periphery thereof are not averaged in terms of their pixel values and become blurred as a result.
Furthermore, owing to limitations imposed by application software in the host computer, there are cases in which the input low-resolution information already exhibits jaggies due to application of nearest neighbor interpolation processing (see FIG. 28).
Means for eliminating jaggies already occurring in input information, as in the case mentioned above, have not been improved upon in the prior art.